test: Add a note explaining in more detail the current and potential benefits of a common TestBase class.

test/test_common/test_base.h

@@ -5,14 +5,27 @@
 namespace Envoy {
 
 // Provides a common test-base class for all tests in Envoy to use. This offers
-// a place to put hooks we'd like to run on every tests.
+// a place to put hooks we'd like to run on every test. There's currently a
+// check that all test-scoped singletons have been destroyed. A test-scoped
+// singleton might remain at the end of a test if it's transitively referenced
+// by a leaked structure or a static.
+//
+// In the future, we can also add:
+//   - a test-specific ThreadFactory that enables us to verify there are no
+//     outstanding threads at the end of each thread.
+//   - a check that no more bytes of memory are allocated at the end of a test
+//     than there were at the start of it. This is likely to fail in a few
+//     places when introduced, but we could add known test overrides for this.
+//
+// Note: nothing compute-intensive should be put in this test-class, as it will
+// be a tax paid by every test method in the codebase.
 class TestBase : public ::testing::Test {
 public:
   static bool checkSingletonQuiescensce();
   ~TestBase() override;
 };
 
-// Templatized version of TestBase.
+// Templatized version of TestBase. See above notes.
 template <class T> class TestBaseWithParam : public ::testing::TestWithParam<T> {
 public:
   ~TestBaseWithParam() { TestBase::checkSingletonQuiescensce(); }